The cost, reliability and performance of gas turbine engines is strongly influenced by the high temperature materials used in their construction. Current aircraft, missile, ground based vehicle and ship propulsion gas turbines, as well as stationary power generation turbines all use metal superalloys to provide needed high temperature performance. Used in polycrystalline or single crystal forms, these metals also impose several important penalties on the overall gas turbine. These alloys are high density metals and thus they contribute to overall engine weight and penalize the engine thrust to weight ratio. The densities of the most significant elements used in these alloys are as follows: nickel 8.90 gm/cm.sup.3 ; chromium 7.19 gm/cm.sup.3 ; columbium 7.19 gm/cm.sup.3 ; cobalt 8.9 gm/cm.sup.3 ; and iron 7.87 gm/cm.sup.3.
This high density in rotating parts is also a major cause of high stresses generated during engine operation. These stresses limit rotor speeds and particularly limit the fatigue life of high temperature discs.
These elements are very costly and, in many cases, are available only from limited sources of supply. As such Co, Cb and less abundantly used elements are referred to as "strategic elements" whose availability in times of peak demand or disrupted supply may be in question.
At very high temperatures these metal superalloys are severely limited due to their propensity to creep under applied stress. Current attempts to increase engine efficiency have caused the operating temperatures to be increased beyond those sustainable by these metals alone. Hence they require cooling by low temperature air which is forced through passages in the blades. This is done at penalty to overall engine efficiency.
Attempts have been made to overcome these deficiencies through the use of ceramic materials such as silicon carbide and silicon nitride. These ceramics are desirable in that they are capable of operating at temperatures well above those of metal superalloys and they are also much lower in density (e.g. 3.2 gm/cm.sup.3). These attempts, however, have been hampered in that these ceramic materials are not fracture tough. They fail readily in the presence of stress concentrations and service induced impact damage.
Another attempt to develop high temperature gas turbine materials has centered on carbon fiber reinforced carbon matrix composites. These materials provide two major advantages over the above metals and ceramics. First, they are very low in density (2. gm/cm.sup.3) and secondly they maintain strength and toughness to extremely high temperatures. Unfortunately carbon is easily decomposed by oxidation at elevated temperatures in a gas turbine and the utility of these materials is thus hinged on the development of coatings and oxidation inhibitors. Because of the extreme thermal fluctuations in a gas turbine the development of these necessary oxidation preventors has been severely limited. In addition, their reliability under stressful operating conditions or during in-service impact damage is extremely questionable. Their fabrication cost is also quite high due to the lengthy high temperature processes required to form their matrix from organic precursors.